But does the painting's enduring appeal lie only in the lady's smile?
To explore this, Hutzler and his colleagues showed volunteers a series of 25, computer-altered faces of attractive women. (The faces were a composite of actual faces so that the features were relatively symmetrical and uniform.)
Each face was shown with three expressions: a smiling mouth, a neutral mouth, or the "Mona Lisa condition," in which the mouth shifted between neutral and smiling.
To do that, researchers tracked each participant's gaze as it shifted from the eyes to the mouth and back again. During the milliseconds it took for the viewers' eye to move, they were basically blind. Researchers alternated between images of the smiling and the neutral mouth during this rapid eye shift, called the saccade, so that viewers were unaware the picture was being changed.
When participants looked directly at the eyes, they caught a slight smile in their peripheral vision. But when their gaze jumped to the mouth, researchers switched to the picture with the neutral mouth -- just like in the Mona Lisa, explained Hutzler.
"Our aim was to simulate this phenomenon of the elusive smile, this smile that you can't catch, to see whether these faces appear more mysterious than non-changing faces," Hutzler said.
Not surprisingly, participants rated the smiling faces as more attractive and trustworthy than neutral faces or the faces in the Mona Lisa condition.
When asked if the faces in the Mona Lisa condition were smiling, beholders were less confident in what they were seeing.
Observers did not rate faces in the Mona Lisa co
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